1. Field of the Invention
The field of the present invention relates to portable electronic devices, including hand held portable lighting devices, such as flashlights, and their circuitry.
2. Background
Various hand held or portable lighting devices, including flashlight designs, are known in the art. Flashlights typically include one or more dry cell batteries having positive and negative electrodes. In certain designs, the batteries are arranged in series in a battery compartment of a barrel or housing that can be used to hold the flashlight. An electrical circuit is frequently established from a battery electrode through conductive means which are electrically coupled with an electrode of a lamp bulb. After passing through the lamp bulb, the electric circuit continues through a second electrode of the lamp bulb in electrical contact with conductive means, which in turn are in electrical contact with the other electrode of a battery. Incandescent lamp bulbs include a bulb filament. Typically, the circuit includes a switch to open or close the circuit. Actuation of the switch to close the electrical circuit enables current to pass through the lamp bulb and through the filament, in the case of an incandescent lamp bulb, thereby generating light.
Traditional flashlights use a mechanical switch to “turn on” the flashlight. This is achieved by mechanically connecting two contacts and allowing current to flow from the positive terminal of the batteries, through the lamp, and back to the negative terminal of the batteries. One of the disadvantages of a mechanical switch is that they are prone to wear and tear as well as oxidation of the elements that physically make and break the circuit. Mechanical switches also do not permit automated or regulated modes of activating and deactivating a flashlight.
Another disadvantage of traditional flashlights is that when they are switched on they instantly allow large amounts of current to flow from the batteries through the lamp filament, thereby stressing the filament. This surge of current occurs because the resistance of the lamp's filament is very low when the filament is cold.
Essentially a lamp filament is a piece of wire that initially acts as a short circuit. The filament resistance builds as the filament heats until the point where light is emitted. Consequently, when the flashlight is initially turned on, a significantly greater amount of current than the bulb is designed to handle flows through the lamp. Although the current surge during this transient stage exceeds the bulb's design limits, the duration of the transient stage is short enough that bulbs generally survive the current surge. Over time, however, this rush of current causes damage to the lamp by stressing the filament and ultimately failure of the lamp filament. Indeed, it is generally during this transient stage that a lamp filament will ultimately fail.
Yet another disadvantage of traditional flashlights is that they are generally powered with alkaline or dry cell batteries. Alkaline or dry cell batteries, when exhausted, are discarded and users have to buy new ones to replace the depleted ones. Replacing batteries is an inconvenience and an additional expense to a flashlight user. Furthermore, alkaline or dry cell batteries are heavy, thereby adding to the overall weight of the flashlight.
Rechargeable lead-acid batteries were developed to replace alkaline and dry batteries. These types of batteries have the advantages of being rechargeable and dischargeable for repeated use. They are, however, relatively large and must be refilled with liquid electrolyte after being used for a period of time. Due to their bulky size and weight, even heavier than alkaline/dry cell batteries, rechargeable lead-acid batteries are usually used with wall-mounted safety lighting fixtures, motorcycles, and automobiles, but are generally not considered suitable for use with portable lighting devices, such as flashlights.
Nickel-cadmium batteries and nickel-metal hydride batteries have been used to replace conventional batteries in flashlights. Nickel-cadmium and nickel-metal hydride batteries have the advantages of being light in weight, convenient for use, and repeatedly rechargeable and dischargeable. However, these batteries have a disadvantage of causing heavy metal pollution. Moreover, the nickel-cadmium and nickel-metal hydride batteries have the so-called battery memory effect. Thus, in order to avoid shortening the life of the batteries, it is necessary to discharge any unused power of these types of batteries before they can be recharged.
An improved rechargeable energy source for portable electronic devices is the lithium-ion battery. Lithium-ion batteries have a higher energy density and a lower self-discharge rate than nickel-cadmium and nickel-metal hydride batteries. Lithium-ion batteries also have a higher energy to weight ratio than nickel-cadmium and nickel-metal hydride batteries. However, a lithium-ion battery can explode if it is charged beyond its safe limits, or if its terminals are shorted together. Further, over discharging a lithium-ion battery can permanently damage the lithium-ion cell. Accordingly, most lithium-ion batteries are made available in a battery pack that includes a built-in protection circuit that has over charge, over discharge, and short circuit protection capabilities. This battery pack protection circuit internally blocks current from flowing from the lithium-ion battery pack when a short is detected. Thus, if there is a short across the recharging contacts for the device, the battery pack protection circuit trips and the electronic device will cease to operate
To avoid such inadvertent interruptions, recharging contacts of portable electronic devices that are powered by a rechargeable lithium-ion batty pack have the contacts in hard to reach or hidden locations. Unfortunately, such a configuration requires the use of plugs, special inserts, alignment tabs or a complex cradle to recharge the batteries. Obstructing access to the recharging contacts is not, however, a viable solution in the case of flashlights or other rechargeable devices where design requirements dictate that the charging contacts or rings be exposed.
If rechargeable lithium-ion batteries were used in a flashlight with exposed charge rings and the user accidentally created a short across the exposed charge contacts with a metal object such as his or her car keys, the lamp would go off until the metal object creating the short circuit is removed. Such inadvertent interruptions may be dangerous when a user is working in an unlit area, especially for law enforcement and emergency response personnel. And, while a simple diode can be placed in the recharging circuit to prevent accidental short circuits from being created across the charging rings or contacts for other rechargeable battery chemistries, such as nickel-cadmium and nickel metal hydride, this solution is not viable for lithium-ion battery packs. A simple diode cannot be used in these circumstances because the forward voltage drop of a diode varies greatly while charging lithium-ion batteries requires very tight control over the termination voltage.
In view of the foregoing, rechargeable lithium-ion battery technology has not been adopted for use in portable electronic devices with exposed charging contacts, such as rechargeable flashlights. A need, therefore, exists for a means of providing improved short circuit protection in rechargeable devices, such as flashlights, having exposed charging contacts. A separate need also exists for a flashlight with improved circuitry that ameliorates one or more of the problems discussed above.